Conventionally, for a decorative paper used for a building interior material (for example, a wall), a surface material of fittings (for example, a door) or furniture, or the like, the surface properties such as the wear resistance and the pollution resistance are required in general. Therefore, for example, Japanese Patent Application Publication (JP-B) No. Sho 49-31033 discloses a decorative paper, which is produced in such manner that a patterned ink layer is formed by printing on a base paper, thereafter an ionizing radiation curable resin paint of an unsaturated polyester pre-polymer is applied on the surface to form a coating layer as the outermost surface layer, and then the coating layer is cross linked and cured by an electron beam to form a surface resin layer.
In such case as described above of providing the decorative paper with the surface resin layer as the outermost layer, which is formed by cross linking a coating layer of an ionizing radiation curable resin comprising a monomer, a pre-polymer, or the like with the ionizing radiation such as an electron beam, a decorative paper having the excellent surface physical properties such as the wear resistance and the pollution resistance can be obtained owing to its high cross linkage.
The decorative paper provided with the surface resin layer as a cross linked coating layer of the ionizing radiation curable resin as mentioned above varies degree of cross linkage in the surface resin layer affected by the ionizing radiation, but it involves the problem of yellowing in any case nevertheless. In particular, the yellowing is conspicuous in the case of using the electron beam.
As to the reason for the yellowing, it was revealed that since the decorative paper uses a paper made of a cellulose based pulp as the base material (base paper), if it is irradiated with the ionizing radiation for cross linking and curing the surface resin layer, a radical (free radical) is generated by the energy (quantum) of the ionizing radiation, and triggered thereby, scission of the principal chain of the cellulose molecule is caused so as to produce a carboxyl group or a carbonyl group so that these groups have the absorption spectra in the short wavelength range of the visible light band (“Influence of the electron beam radiation on the paper quality (second edition)/novel method for providing the electron beam resistance to the paper”, Japan Tappi Journal vol. 51, No. 6, published in June, 1997, page 945 to page 954).
Moreover, in the above-mentioned report, as the countermeasure for the yellowing, proposed is impregnation of the paper with an alcohol solution of an electron beam reactive monomer such as a tetraethylene glycol dimethacrylate (TEGDMA), a polyethylene glycol dimethacrylate (PEGDMA), or the like for allowing the monomer to scavenge the radical generated at the time of the electron beam irradiation, and for cross linking or graft polymerizing the monomer with a cellulose molecule at the time of the electron beam irradiation.
This method is indeed one of the effective means for prevention of the base paper yellowing at the time of the ionizing radiation irradiation.
However, since a special chemical is used for the impregnation, cost of the base paper becomes higher thus being problematic. Moreover, it is effective for prevention of the yellowing, but not complete, and the yellowing can be conspicuous depending on amount of the electron beam irradiation, or depending on the kind of the design to be printed. The yellowing becomes problematic particularly in the following two cases.
(1) The case that the design and appearance of the decorative paper inherently cannot tolerate the yellowing.
Examples include: the case of requiring a pure white appearance; the case that the change of the hue itself becomes conspicuous by the yellowing such as pale blue, red, green, or the like; and the case that the base paper loses harmony with the patterned ink layer or the balance of the color mixture by the yellowing so that the designed appearance itself is changed.
(2) The case that the base paper yellowing itself can be tolerated, but change of the hue before and after the ionizing radiation irradiation becomes the obstacle in terms of the color tone administration of the decorative paper.
For example, in general, a process of printing a patterned ink layer to the base paper and a process of coating a surface resin layer and irradiating with an ionizing radiation are executed in different processing machines. In this case, the patterned ink layer is printed by administering the color tone so as to match the color tone with the standard color immediately after the printing process. However, in the case that the yellowing is generated by the ionizing radiation in the following surface resin layer coating process, the once matched color tone becomes unmatched. Moreover, the degree of the yellowing generates irregularity depending on the irregularity of the ionizing radiation irradiation condition, the irregularity of the base paper lot, the environmental humidity, or the like. Therefore, it is very difficult that the color tone is matched preliminarily at the printing process so as to be matched with the standard color after having the yellowing at the final process.
That is, an object of the present invention is to have the yellowing of the base paper by the ionizing radiation at the time of cross linking the coating layer inconspicuous even in the case that a decorative paper is designed to have a surface resin layer made of a cross linked coating layer of an ionizing radiation curable resin.